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Re: [Xen-devel] [PATCH v2] xen: grant-table: Simplify get_paged_frame

Hi Roger,

On 19/09/17 12:40, Roger Pau Monné wrote:
On Tue, Sep 19, 2017 at 10:44:53AM +0100, Julien Grall wrote:

On 19/09/17 08:13, Roger Pau Monné wrote:
On Mon, Sep 18, 2017 at 06:32:22PM +0100, Julien Grall wrote:
Hi Roger,

On 18/09/17 17:58, Roger Pau Monné wrote:
On Mon, Sep 18, 2017 at 05:27:52PM +0100, Julien Grall wrote:
The implementation of get_paged_frame is currently different whether the
architecture support sharing memory or paging memory. Both
version are extremely similar so it is possible to consolidate in a
single implementation.

The main difference is the x86 version will allow grant on foreign page
when using HVM/PVH whilst Arm does not. At the moment, on x86 foreign pages
are only allowed for PVH Dom0. It seems that foreign pages should never
be granted so deny them

The check for shared/paged memory are now gated with the respective ifdef.
Potentially, dummy p2m_is_shared/p2m_is_paging could be implemented for

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>


Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>

       Changes in v2:
           - Deny grant on foreign page (aligned with the ARM code)
           - Use #ifdef rather than #if defined
           - Update commit message
           - Fix typo in the title

get_page_from_gfn will be able to get reference on foreign page and as
per my understanding will allow to grant page on foreign memory.

This was not allowed with a simple get_page(...) on the ARM
implementation (no sharing nor paging supprot) but is allowed on the x86
implementation due to get_page_from_gfn.

On x86, foreign pages are currently only allowed for PVH dom0, so I
think it is not a big deal for now.

On Arm, foreign pages can be present on any domain. So this patch would
permit grant on foreing pages.

This patch will deny granting foreign pages. Jan Beulich is happy with
it. Any other opinions?

Won't this break QEMU running in stub domains?

I haven't tested it, but I'm afraid QEMU running in a stub domain
might try to grant a foreign frame. Ie: the emulated network code in
QEMU might try to grant a foreign frame in order to forward operations
from emulated devices to PV frontends.

I don't think it will break any existing setup because foreign mapping are
only allowed for the hardware domain (see p2m_add_foreign).

IIRC this only applies to auto-translated (HVM) domains, not to PV
domains. QEMU stubdomains are PV guests.

Still, p2m_map_foreign can only be used for auto-translated domains.

Oh, so you are limiting this only for auto-translated guests? Should
have looked more closely at the commit message.

That's right. I am happy to update the commit message if it is not clear enough.


Julien Grall

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